Manchester: Romelu Lukaku was stretchered off after the Manchester United striker clashed heads with Southampton's Wesley Hoedt in the teams' Premier League clash on Saturday.
In the absence of Lukaku, Mourinho's men were unable to break down Southampton and had to settle for their third successive league draw.
United end a frustrating December trailing 14 points behind leaders Manchester City, who face Crystal Palace on Sunday.
Lukaku crashed into Hoedt as the pair contested an aerial challenge in the first half of the encounter at Old Trafford.
The Belgium international lay prone on the pitch barely moving while he was treated by medical staff for five minutes.
Lukaku remained conscious and was eventually carried off to applause from the United fans.
His injury continued a difficult period for Lukaku, who has scored just four times in his last 19 appearances for Jose Mourinho's side.
Lukaku, who joined United from Everton for £75 million ($101 million) in the close season, had started the campaign with 11 goals in 10 games.